Welcome to Sitepoint, Andy.
I am retitling AND moving your post , as it really has little to do with CSS and more with WP functionality (worse, WP plug-ins)
CSS is about styling the HTML ( so that would me more of a WP skin, or maybe a style sheet of a theme) , themes/ and plug ins do content control by actually outputting the HTML.
I really avoid tend to avoid plugins ( because I can code my own), so I cant say I am familiar with he one you mentioned. But from a coders perspective, I would guess that the pluggin hid your titles in one of 3 ways:
1) attaching an action_filter to content; but if that the case even simply deactivating the plug in would have nullified any of its effects
2) it could have created a separate DB table and stored your headlines there ( which would mean the headlines would be missing you go to edit them too) ; This is stupid, but then again that's why I hate 3rd party plug ins. In which case hastily removing the plug in may have now left your posts headlineless
3) it could have 'marked off' the headlines in the native WP DBs, so when you go to edit the posts you might still see the headline but the headline would act as if it was unpublished.
these is just speculating.
BTW, HOW did you remove the plug-in? Did you deactivate it properly from the dashboard, or did you just remove the files?